In recent years, there has been a reduction in the power supply voltage of transmitter optical sub-assemblies (TOSAs) employing a laser diode, which are incorporated in communications devices using optical fibers (see, e.g., Japanese Laid-Open Patent Publication No. 2002-111118). As a result, the transmission line to the laser diode has been adapted to provide 25 O impedance matching, instead of 50 O impedance matching. Furthermore, to further reduce the power supply voltage, there is a tendency to reduce or eliminate the matching resistance of the laser diode side while still using a 25 O transmission line. That is, although a standardized 25 O transmission line is used, the total matching resistance of the laser diode side is reduced to 7 O, which is the value of the internal resistance of the laser diode. This leads to mismatches among the matching resistance value of the laser diode side, the characteristic impedance of the transmission line, and the matching resistance value of the drive circuit side, resulting in degraded reflection characteristics and increased noise and jitter in the output optical waveform signal.
In order to increase the resistance to external noise, the drive circuit has been adapted to perform an active matching operation. An active matching operation is an operation performed to cancel noise coming from the laser diode. This operation allows the optical waveform signal to be shaped such that it includes reduced noise and jitter.
In a conventional drive circuit, an emitter follower circuit is provided on the input side of the final stage amplifier circuit so as to form a feedback path for active matching operation. However, this feedback path is long, resulting in an increased time constant. As a result, the response speed of the active matching operation to cancel noise produced by the laser diode is low, which prevents the noise from being reliably canceled. Furthermore, it is very difficult to match these circuits in a high frequency range (e.g., in the 10 Gbps region).